Low voltage fully differential operational amplifiers

Abstract: A low voltage fully differential operational amplifier with improved common mode circuitry for low supply voltage applications. The amplifier includes inverting and non-inverting inputs and dual-ended inverted and non-inverted outputs. The amplifier also includes source transistors for pushing the output and sink transistors for pulling the output to obtain particular output voltage swing. Connections between the inputs and outputs include DC biasing circuitry. In low supply voltage applications, voltage drop due to transistors of amplifier circuitry can be significant and can affect possible range of output voltage swing. The DC biasing circuitry includes four identically sized n-channel transistors; three of the n-channel transistors receive a second bias current input and one of the n-channel transistors receives a first bias current input, identical to the first; three identically sized n-channel cascode transistors are connected to the n-channel transistor that receives the first bias current input and to two of the n-channel transistors receiving the second bias current input; the other n-channel transistor receives the second bias current input and is connected to a correcting transistor of select size and characteristics. The correcting transistor serves to maintain mirrored currents between each of the source transistors and each of the sink transistors to eliminate the implications of inequivalent voltage drop across corresponding series of transistors in the amplifier so that the range of output voltage swing, even in low supply voltage applications, closely approaches the value of the power supply voltage. The common mode circuitry of the amplifier provides a high level of common mode signal reconstruction. The common mode circuitry accepts inputs of a signal indicative of the difference between the outputs and an analog reference voltage input. Based on those inputs, the common mode circuitry either pushes or pulls the output stage of the amplifier to draw the outputs close to the voltage of the analog reference input. The common mode circuitry is designed to also mirror currents in a manner which eliminates current mismatch between the amplifier core and the common mode circuitry, which mismatch can otherwise cause reconstruction errors in the voltages of the outputs. The common mode circuitry, together with the DC biasing circuitry, maximize current matching between the amplifier core and the common mode circuitry.

Description: The invention relates to fully differential amplifiers for use as operational amplifiers, and more particularly, to fully differential operational amplifier for low power supply operation, having a unique DC biasing scheme and incorporating a new common mode reconstruction technique. Fully differential amplifiers are common components of electronic devices. A fully differential amplifier is an amplifier with two inputs and two outputs. The outputs are amplified versions of the inputs. These amplifiers serve to amplify the two input signals to the amplifier, yielding dual amplified outputs. Fully differential amplifiers are important in many applications where weak signals (for example, in low supply voltage arrangements) may be contaminated by "pickup" and other miscellaneous noise. In the case of many low supply voltage operations, like battery powered or portable communication devices, if a single-ended input (i.e., a single, individual input signal with an absolute value) is input and used by an amplifier, undesired "pickup" and noise which are input with that signal may be also picked up and amplified along with the desired input signal. In that event, output from the amplifier will vary from desired output depending upon the particular "pickup" or other noise. If, on the other hand, the amplifier is a fully differential amplifier and so has dual inputs and dual outputs, then both inputs to the amplifier pick up the same "pickup" or noise signals at the same time and those "pickups" or noise signals are amplified along with the desired input signals to give the respective outputs. In a difference determination between the two outputs, ther the amplified "pickup"/noise components of the outputs cancel each other and there is no resulting differential amplification effect from or of these undesired signals. Fully differential amplifiers generally include common mode circuitry. In the typical fully differential amplifier, the common mode circuitry receives a reference analog voltage signal and serves to pull or push the outputs of the fully differential amplifiers as necessary to bring the voltages of the outputs near that of the reference analog voltage signal. The process of pulling or pushing to bring the two output signal voltages near the reference analog voltage is referred to as "common mode reconstruction". Effective common mode reconstruction is a desired attribute of fully differential amplifiers. In simplistic form and for purposes of a better understanding hereof, a single stage fully differential amplifier may be thought of as consisting of two inputs, two outputs, and some means for common mode reconstruction. Each of the outputs is taken off a transistor collector with respect to ground. The amplifier circuit responds differently to common-mode signals than to non common-mode (or "differential") signals. Common-mode signals drive each of the inputs in phase with equal-amplitude voltages. Differential signals, on the other hand, are applied to either of the inputs, not both, and so drive only the particular input to which the signal is applied. In operation, the fully differential amplifier takes the differential signal inputs and creates differential outputs, the phase and amplitude of each being dictated by the respective input signal and feedback considerations; the amplifier takes the common-mode signal inputs and creates two outputs in respect of those common-mode inputs, which are same-phase and equal-amplitude voltage. Where both differential and common-mode signals are simultaneously input to the two inputs, each output of the fully differential amplifier is dictated by the two components of the respective input, i.e., the common-mode signal component and the differential signal component. In a difference determination between the two outputs of the fully differential amplifier, the components of each output attributable to common-mode signals may be cancelled. Fully differential amplifiers, like all solid-state amplifiers, require some form of bias. At the minimum, the base-source junction of any solid-state amplifier must be reverse biased. Other biasing may be appropriate to obtain desired operation of the amplifier, such as desired output voltage swing and others. The desired bias, in any event, is produced by applying voltages to the corresponding transistor elements through bias networks. Biasing networks may typically serve a number of purposes, and, in any case of a biasing network, biasing for one purpose may affect other aspects of amplifier circuit operation. One biasing technique, sometimes referred to as matched base-emitter biasing, can be used to make what is called a current mirror. A current mirror is important in fully differential amplifier devices in a number of instances where a current source is needed. A common arrangement for a current mirror is sometimes referred to as a Wilson mirror. The Wilson mirror is a pair of gm p-channel or n-channel MOSFET devices connected in series. This arrangement has one leg carrying a reference current which is mirrored by the two transistors in series in the other leg. This technique works to sufficiently mirror current in most cases, however, in low power applications, a relatively significant voltage drop occurs across the two transistors in series. That voltage drop takes up a significant amount of the possible output voltage swing of the amplifier, so the actual output voltage swing is often within a narrower range than the range of operating voltage available from the power supply. In addition to biasing, it is important in fully differential amplifiers to have operational characteristics sufficient to prevent conversion of common-mode signals (i.e., "pickup" or other noise) into difference signals (which produce an output). As previously discussed, fully differential amplifiers operate by producing dual outputs in response to respective dual inputs. Both leads of these amplifiers will pick up the same "pickup" and other miscellaneous noise (i.e., common-mode signals) at the same time, which will appear amplified in the respective outputs. The outputs are then typically subjected to a difference determination in which the "pickup"/noise signals appearing amplified in each of the outputs is cancelled. Fully differential amplifiers are operational amplifiers. They have all the same characteristics with the exception of having two outputs rather than one. In effect, fully differential operational amplifiers behave by "looking" at a reference voltage and swinging the output terminals around so that the voltages of the outputs draw close to that of the particular amplifier power supply voltage. In an ideal arrangement, the swing of voltage of each of the outputs may be as great as the range of power supply operating voltage. For example, if a power supply yielding a 0 to 5 volt supply voltage range operates a fully differential amplifier, then the voltage of each of the outputs can theoretically swing between 0-5 volts. Likewise, in the case of a power supply yielding a 0 to 2.7 volt supply voltage range for a fully differential amplifier, a 0-2.7 volt swing is theoretically possible for each of the outputs. So these are the ideal cases for a 0-5 supply voltage and a 0-2.7 supply voltage, respectively. In the real world these ideal conditions cannot be absolutely met. The real world fully differential operational amplifier will be operable around some range at about the mid-voltage of the power supply operating voltage range and the voltage swing of each of the outputs will be around this mid-voltage plus or minus some ΔV, which is something less than the theoretical output voltage possible given the range of supply voltage available from the power supply. In practice, fully differential operational amplifiers typically work best at around a mid voltage of the two outputs, which is considered the common level voltage. In the present specification, we describe a low voltage fully differential operational amplifier having improved common mode circuitry. The amplifier is designed to work with low supply voltages and yet meet the output range performance requirements found in higher voltage applications. The amplifier includes an improved DC biasing scheme and improved common mode circuitry providing more effective common mode reconstruction. These features allow the outputs of the amplifier to swing over a substantially greater range, even in low power supply applications and solve many diverse problems found in the prior technology, for example, problems of gain-band width, output swing, stability, common mode reconstruction, and manufacturability. An example of a prior art amplifier which is designed to work with such low supply voltages is given in : 1993 IEEE International Symposium on Circuits and Systems, vol. 2, 3 May 1993, Chicago, US, pages 1455 to 1458, XP 379250, "Designing Operational Transconductance Amplifiers for Low Voltage Operation" by P.J. Crawley et al. The invention is defined by the appended claims. According to one aspect of the invention, the fully differential operational amplifier comprises an inverting input, a non-inverting input, a first output, a second output, a reference voltage input, means for amplifying the inverting input and the non-inverting input to yield the first output and the second output, and means for common mode reconstruction which receives the reference voltage input and the first output and second output and pushes or pulls each of the first output and the second output to draw the voltage of the outputs substantially near the voltage of the amplifier power supply voltage. In another aspect, the means for common mode reconstruction includes means for differential determination, connected to the first output and the second output, for determining any difference between the outputs, means for pushing the outputs high, selectively operable in response to any difference between the outputs, and means for pulling the outputs low, selectively operable in response to any difference between the outputs. In yet another aspect, the means for common mode reconstruction includes common mode circuitry. In even another aspect the fully differential operational amplifier further comprises at least one cascode means connected to the means for pushing and at least one cascode means connected to the means for pulling. In a further aspect, the means for common mode reconstruction includes DC biasing circuitry. In even another aspect, the means for common mode reconstruction comprises an input of an externally supplied analog ground reference voltage of about the mid-point of the range of power supply voltage and an input of a difference signal determined as a difference of the first output and the second output, the inputs each being directed to a gate of a p-channel transistor, each of the p-channel transistors being connected to series connected n-channel transistors, each of said n-channel transistors having a common drain, and series connected p-channel transistors connecting between the inputs. In yet a further aspect, the means for pushing is at least one source transistor and the means for pulling is at least one sink transistor. In even another aspect, the DC biasing circuitry establishes bias voltages on the means for pulling, thereby maintaining the means for pulling in saturation for substantially the entire range of output. In yet another aspect the means for amplifying includes DC biasing circuitry, the DC biasing circuitry including four identically sized n-channel transistors, three of the n-channel transistors receiving a second bias current input and one of the n-channel transistors receiving a first bias current input, and three identically sized n-channel sink transistors connected to the n-channel transistor receiving the first bias current input and to two of the n-channel transistors receiving the second bias current input, the other of the n-channel transistors receiving second bias current input and being connected to a select transistor having desired characteristics to maintain current mirroring between each of the at least one source transistors and between each of the at least one sink transistors, thereby causing the at least one source transistors and the at least one sink transistors to remain in the saturation region for the entire range of swing of the outputs. In even a further aspect, the DC biasing circuitry establishes bias voltages on said means for pulling, thereby maintaining the means for pulling in saturation for substantially the entire range of voltages of the outputs. In yet a further aspect, each of the p-channel transistors are biased by at least one applied bias voltage and at least one of each of the series connected n-channel transistors are biased by at least one different applied bias voltage. In yet even a further aspect, the DC biasing circuitry selectively establishes bias voltages on the means for pulling, thereby maintaining the means for pulling in saturation for substantially the entire range of voltages of the outputs, and on the means for pushing, thereby maintaining the means for pushing in saturation for substantially the entire range of voltage of the outputs. In another aspect, the fully differential operational amplifier further comprises at least one cascode means connected to the means for pushing and at least one cascode means connected to the means for pulling, wherein the DC biasing circuitry also establishes bias voltages on the at least one cascode means connected to the means for pushing and on the at least one cascode means connected to the means for pulling. In even another aspect, the at least one of each of the series connected n-channel transistors are biased by the bias voltages established by the DC biasing circuitry. In a further aspect, the means for common mode reconstruction includes common mode circuitry with currents substantially mirroring those of the means for amplifying. In even a further aspect, the other of the n-channel transistors receiving the second bias current input and the select transistor are connected to the at least one of each of the series connected n-channel transistors to maintain current mirroring between each of the n-channel transistors of the DC biasing circuitry and the series connected n-channel transistors of the means for common mode reconstruction. In another aspect, the common mode circuitry comprises an input of an externally supplied analog ground reference voltage of about the mid-point of the range of power supply voltage and an input of a common mode output voltage from the amplifier, the inputs each being directed to a gate of a p-channel transistor, each of the p-channel transistors being connected to series connected n-channel transistors, each of said n-channel transistors having a common drain, and series connected p-channel transistors connecting between the inputs. In another aspect of the invention, improved DC biasing circuitry and common mode circuitry are provided for a fully differential operational amplifier having dual inputs and dual-ended outputs, and further having p-channel source transistors and n-channel sink transistors, the DC biasing circuitry causing the output to be pushed high when the voltage differential between the dual inputs is positive to bring the voltage level of the outputs near the value of the power supply voltage and to be pulled low when the voltage differential between the dual inputs is negative to bring the voltage level of the outputs near the value of the ground, and the common mode circuitry receiving an input of an externally supplied analog ground reference voltage of about the mid-point of the range of power supply voltage and an input that is the differential between the outputs from the amplifier and substantially mirrors currents of the common mode circuitry in order to reconstruct the outputs to account for voltage errors attributable to common mode signals input to the amplifier via the inputs and due to voltage drop attributable thereto within the amplifier. In yet another aspect, there is provided a method of common mode reconstruction in a differential operational amplifier having dual inputs and dual ended outputs, the amplifier comprised of sinking transistors and sourcing transistors and corresponding biasing transistors, comprising the steps of inputting an externally supplied analog ground reference voltage of about the mid-point of the range of power supply voltage for the amplifier to a gate of a first p-channel transistor, inputting a difference signal based on comparison of the outputs, from the amplifier, to a gate of a second p-channel transistor, connecting each of the first p-channel transistor and the second p-channel transistor to series connected p-channel transistors, connecting the first p-channel transistor to series connected n-channel transistors, each of said n-channel transistors having a common drain, connecting the second p-channel transistor to series connected n-channel transistors, each of said n-channel transistors having a common drain, connecting each of the series connected n-channel transistors to a common ground, and connecting the amplifier at a drain of the p-channel transistor. Preferably, the method further comprises the step of connecting the amplifier with DC biasing circuitry. Preferably, the DC biasing circuitry accounts for voltage differential between dual biasing currents across biasing transistors of the biasing circuitry to yield substantially identical biasing currents to all of the sinking transistors and substantially identical biasing currents to all of the sourcing transistors and all of the sinking transistors of the amplifier. Preferably, the method further comprises the step of connecting cascode transistors to each of the sourcing transistors and the sinking transistors. In a further aspect, the power supply voltage ranges from about 0 volts to about 2.7 volts. In even a further aspect, the power supply voltage ranges from about 0 volts to about 5 volts. For a more complete understanding of the present invention and for further objects and advantages thereof, reference may now be had to the following description in conjunction with the accompanying drawings, in which, by way of example only: FIG. 1 is a detailed schematic representation of a prior art, single stage fully differential operational amplifier, utilizing classical methods for biasing and current mirroring and for common mode circuitry; FIG. 2 is a detailed illustration of the DC biasing and current mirroring circuitry of the prior art single stage amplifier of FIG. 1; FIG. 3 is a detailed schematic representation of the preferred embodiment of the low voltage fully differential operational amplifier circuit of the present invention, having unique DC biasing and current mirroring schemes and improved common mode reconstruction circuitry; FIG. 4 is a detailed illustration of the DC biasing and current mirroring circuitry of the preferred embodiment of the amplifier circuit of the present invention of FIG. 3; FIG. 5 is a detailed schematic representation of the complementary version single stage amplifier of that shown in FIG. 3; FIG. 6 is a detailed schematic representation of a two stage Class A differential operational amplifier incorporating the DC biasing and current mirroring schemes and the improved common mode reconstruction circuitry of the present invention; and FIG. 7 is a detailed schematic representation of a two stage differential operational amplifier incorporating the DC biasing and current mirroring schemes and the improved common mode reconstruction circuitry of the present invention, and having pole-zero compensation. In order to understand the principles and advantages of the present invention, it is helpful to first consider a prior art single stage differential operational amplifier and its operation and characteristics. Referring first to FIG. 1, such a prior art single stage differential amplifier 2 is shown. The amplifier 2 generally includes a single stage amplifier section, a network of DC biasing circuitry, and a common mode reconstruction circuitry stage. Inputs to the single stage amplifier consist of an inverting input 12 and a non-inverting input 10. The amplifier 2 has dual-ended non-inverting and inverting outputs 14,15, respectively. The amplifier section also includes an arrangement of p-channel transistors 20,22,24,26,28,30 and an arrangement of n-channel transistors 32,34,36,38. Continuing to refer to FIG. 1, the amplifier section includes an input stage and an output stage. The input stage of the amplifier 2 includes the inputs 10,12, transistors 24,26, and biasing currents 70,72 having bias voltages 70a,72a for those transistors. The output stage of the amplifier includes outputs 14,15 (positive and negative, respectively) transistors 20,22,32,34 and 28,30,36,38, and biasing currents 66,68 having bias voltages 66a,68a, respectively. As will be apparent to those skilled in the art, the p-channel transistors 20,28 serve as current sources with cascode devices 22,30 and the n-channel transistors 32,36 serve as current sinks with cascode devices 34,38. Still referring to FIG. 1, the network of DC biasing circuitry of the prior art single stage amplifier 2 includes p-channel transistors 50,52 and n-channel transistors 54,56,58,60,62,64. Current inputs 66,68 to these n-channel transistors 54,56,58,60,62,64 of the DC biasing circuitry are seen to also appear as bias voltage inputs 66a,68a to the n-channel cascode devices 34,38. The current inputs 70,72 to the p-channel transistors 50,52 are seen to appear as bias voltage inputs 70a,72a to the input stage transistors 24,26 and the p-channel transistors 20,22,28,30. Continuing still to refer to FIG. 1, the prior art amplifier 2 is seen to also include certain common mode circuitry. The common mode circuitry of the amplifier 2 is comprised of n-channel transistors 82,80 and p-channel transistors 84,86,88,90. The common mode circuitry also includes an input 92 which serves to accept an externally supplied analog ground reference voltage, typically around the mid-point of the supply voltage range. Another input 94 is the common mode signal which is generated from the amplifier section outputs 14,15, which generation is typically by means of a resistor divider between the outputs 14,15 or some switched capacitor divider technique between the outputs 14,15. In any event, the purpose of the common mode circuitry is to generate a signal 93 which either pushes or pulls the outputs 14,15 so that the voltages of the outputs 14,15 are drawn to that of the reference input 92. Further referring to FIG. 1, the prior art amplifier 2, with differential or common mode circuitry, operates as follows: Generally, when there is a difference in the voltages of the inputs 10,12, a current imbalance is created in both output stages to either the source or sinking transistors so that more current is being sourced or sinked, depending on the polarity of the imbalance. For example, if the two inputs 10,12 have identical voltages, the differential pair is split evenly and the circuitry creates a current sink that has the same current as the current source. Then, as one potential comes up and the other potential comes down, a current imbalance results in both output stages such that there's more current being sourced or sinked, depending upon the direction of the imbalance. Along with this general operation of the amplifier and biasing circuitry of the prior art amplifier 2, the common mode circuitry of the amplifier serves to supply a common mode reconstruction or correction voltage signal to the outputs of the amplifier. The illustrated prior art common mode circuitry operates by comparing a DC level generated from the amplifier outputs 14,15 as input 94, as previously described, to an externally supplied analog ground reference voltage input 92. Based on these two inputs 92,94 to the common mode circuitry, the common mode circuitry generates a signal 93 which either pushes or pulls the outputs 14,15 so that the voltages of the outputs 14,15 are drawn to that of the analog ground reference voltage input 92. Continuing to refer to FIG. 1 now in conjunction with FIG. 2 which more particularly illustrates the DC biasing and current mirroring circuitry of the prior art amplifier 2, the classical manner for sizing this prior art biasing network is to set all of the biasing network n-channel transistors 54,56,58,60,62,64 equal in size and to source four times the drain current into transistor 56. This technique creates a V ds across transistor 56 equal to V t + 1.4 V dsat and since the V t of transistor 56 and transistor 60 are similar (the V t of transistor 60 will actually be slightly higher due to body effects), the gate to source drop across transistor 60 will cause transistor 58 to be biased with a V ds of about 1.3 V dsat which will keep transistor 58 in saturation. Still referring to FIG. 1 in conjunction with FIG. 2, this biasing and current mirroring technique of the prior art is fundamentally problematic because the mirroring transistor 58 in one leg of the current mirror circuitry has a smaller voltage drop V ds than the single transistor 54 in the other leg. This difference in voltage drop between the two legs of the mirroring circuitry causes a mismatch in the current mirror output. The mismatch, though small, can be significant in cases in which the entire device operates at low voltage, and, in those cases, the mismatch can affect the entire operating characteristics of the operational amplifier. As is clear, the mismatch can affect voltages to the common mode circuitry, causing the common mode circuitry to operate less effectively. In addition to differences in voltage drop across transistors in legs of the mirroring circuitry, the transistor V ds and, thus, the current mismatch in the mirroring circuitry of the prior art design, can vary as a result of process, temperature, supply voltage, and reference current changes. Even further, changes in transistor sizes employed in the circuitry can result in significant mismatch problems because there is no easy way of changing transistor sizes in the prior art design and still effecting desired current mirroring. Thus, as is and will become more apparent, the present invention, which substantially reduces the mismatch in a wide variety of circumstances and which includes improved common mode circuitry that takes advantage of the mismatch reduction to improve common mode reconstruction, is a significant improvement in the art, in particular, in low supply voltage applications. Now referring to FIG. 3, a preferred embodiment of a single stage, fully differential amplifier 4 of the present invention is illustrated. This amplifier 4 incorporates an improved common mode circuitry technique. This amplifier 4 is also designed to have a minimum voltage drop across the p-channel source transistors 120,124,128 and n-channel sink transistors 132,136, so that the outputs 114,115 can swing over a greater range. The improved common mode circuitry is especially effective when employed in conjunction with this particular amplifier 4 design. The improved common mode circuitry and the whole of the amplifier 4 design, together with the advantages of each, will be more fully understood through the following descriptions. Continuing to refer to FIG. 3, in general, the amplifier 4 is organized similar to the prior art amplifier 2 (shown in FIG. 1) in many respects. The present invention includes both a single stage folded cascode amplifier section and a network of DC biasing circuitry. The single stage folded cascode amplifier section has inputs of an inverting input 112 and a non-inverting input 110. The amplifier 4 also has dual-ended non-inverting and inverting outputs 114,115, respectively. The amplifier section consists of an arrangement of p-channel transistors 120,122,124,126,128,130 and an arrangement of n-channel transistors 132,134,136,138. Still referring to FIG. 3, the DC biasing circuitry of the amplifier 4 is seen to consist of p-channel transistors 170,172,174,176 and n-channel transistors 150,152,154,156,158,160,162,164. Inputs to the DC biasing circuitry include dual biasing circuitry inputs 196,198 of identical current. The function of the DC biasing circuitry in this preferred embodiment, and a difference in this amplifier 4 compared to the prior art, is that the DC biasing circuitry of the present amplifier serves to establish bias voltages on the current sources, transistors 120,124,128, on the current sinks, transistors 132,136, and on the cascode devices, transistors 122,126,130,134,138. In this manner, the current mirroring between sourcing transistors 120,128 and sinking transistors 132,136 maintains those transistors in the saturation region for all values of output swing. Continuing still to refer to FIG. 3, another very significant difference in this amplifier 4 compared to the prior art, is the improved common mode circuitry design employed in this amplifier 4. This common mode circuitry, in particular, is quite effective in taking advantage of the particular biasing concepts employed in the invention. The common mode circuitry yields signal 193 which is a voltage input to gate for n-channel transistors 132,136 of the amplifier section. The common mode circuitry consists generally of p-channel transistors 218,220,222,224 and n-channel transistors 210,212,214,216. The p-channel transistors 222,224 are biased by bias voltages 202,204. Inputs to the common mode circuitry include an externally supplied analog ground reference voltage input 242 to p-channel transistor 218 and a common mode signal input 244 to p-channel transistor 220. As previously described with respect to the prior art, the common mode signal input 244 is generated in a conventional manner, such as by means of a resistor divider between the amplifier 4 outputs 114,115 or some switched capacitor divider technique between those outputs 114,115. Still referring to FIG. 3, the amplifier functions as follows: Generally, when the voltages of the inputs 112,110 are equal and within the operating range of the amplifier, the current in the output stage, i.e., transistors 120,122,132,134 and 128,130,136,138, is balanced. When the inputs 112,110 have a differential voltage between them, the output stage has a current imbalance and either pulls high or low. The external load capacitance and feedback serve to give the amplifier 4 compensation and closed loop functionality. One significant aspect of the amplifier 4 which gives the amplifier 4 its favorable characteristics for low supply voltage applications is the particular implementation of the biasing circuitry formed on sinking transistors 132,136, on sourcing transistors 120,124,128, and on cascode transistors 122,126,130,134,138. This circuitry allows the amplifier 4 to exhibit favorable operating characteristics, even in low supply voltage applications. Through these particular aspects, the amplifier's transistors are maintained in saturation and the use of the cascoded load improves power supply rejection. The common mode circuitry of the amplifier 4 takes particular advantage of these characteristics and provides certain additional favorable aspects to the configuration. In particular, the common mode circuitry accepts a DC level input 244 that is the mid-point of the two outputs 114,115 of the amplifier 4. The common mode circuitry then compares that DC level input 244 with an analog reference voltage input 242 that is equal to the mid-point level of the supply voltage. If the DC level input 244 and reference voltage input 242 differ, a correction for that difference results in a common mode signal 193 which adjusts the amplified outputs 114,115 so that the common mode is reconstructed accordingly, i.e., either pushed or pulled to bring the voltages of the amplified outputs 114,115 very near that of the reference voltage 242. In connection with the entire amplifier, note that the common mode circuitry employs similar biasing schemes to that of the DC biasing circuitry. This serves to set all V ds and V gs potentials equal, thereby maximizing the current matching between the main amplifier and the common mode circuitry. These characteristics of the amplifier 4 can be better understood by considering the particular circuitry and variables involved in a typical application of the amplifier 4. Referring now to FIG. 4, an illustration of the DC biasing circuitry of the present invention amplifier 4 is shown. This new biasing circuit creates equal V ds voltages across transistors 150,158 that results in more accurate current matching and less changes for variations in process, temperature, supply voltage, or I ref current. This DC biasing circuit requires that transistors 152,156,160 (and also transistor 164 in the amplifier circuit shown in FIG. 3) be sized the same, or as unit multiples, and transistors 150,158 be sized the same, or as unit multiples. The size of transistor 154 is set so that the desired V ds is derived across transistors 150,158 (and transistor 162 in FIG. 3) so they remain in saturation (typically 1.3 V dsat ). As a result of this technique, all of the critical transistors are the same size as or unit multiples of each other (except for transistor 154 which sets the V ds for transistors 150,158) (and 162 in FIG. 3), and the cascode and sinking devices may be varied in size and still maintain close matching. Continuing to refer to FIG. 4 and now in conjunction with FIG. 2, in effect, the addition in the present invention of transistor 154 to the DC biasing circuitry serves to set the potential across transistor 150 equal to the potential across transistor 158, to get a true current mirror. The middle stage of the biasing circuitry, i.e., transistors 154,156, establishes the voltages on the cascode devices, so that the gate to source drop of transistor 152 will drive transistor 150 to an appropriate V ds voltage. Further note that the gate of transistor 150 is, instead of being tied back to drain, tied to the other side of the cascode device so that it picks up the potential, i.e., the additional potential, so that the currents will be mirrored correctly and will have the same drain to source potential. Still referring to FIG. 4 in conjunction with FIG. 3, in the single stage amplifier section of the amplifier 4, this same biasing technique is employed to bias 196 the n-channel cascode devices 134,138 and to set up the bias 198 for the n-channel sink devices 132,136. Note also that, in a similar fashion, currents are mirrored so that bias voltage 204 biases the p-channel devices 120,124,128 and bias voltage 202 sets up the bias voltage for the cascode transistors 122,126,130. Referring further to FIG. 4 in conjunction with FIG. 3, a similar biasing technique is also employed in the common mode circuitry. Transistors 222,224 of the common mode circuitry are biased by voltages 202,204, respectively, the same voltages which bias the p-channel source and cascode transistors 120,122,124,126,128,130 of the amplifier section. Note also that, as with transistor 150 in the DC biasing circuitry, the gates of transistors 210,212 are tied to the other side of the cascode devices 214,216, respectively. With this new common mode circuitry design, current mismatch between the common mode circuit and the amplifier core can be substantially reduced, thus, minimizing offset errors in the common mode output voltage 244. The biasing scheme, used in connection with this improved common mode circuitry, minimizes those errors by setting all of the V ds and V gs potentials the same, thereby maximizing the current matching between the main amplifier and the common mode circuitry. Next referring to FIG. 5, a complementary, inverted version of the low voltage fully differential operational amplifier with improved common mode circuitry of the present invention is shown. This complementary, inverted version performs in essentially the same manner as the embodiment depicted in FIG. 3, but is the exact complement of the FIG. 3 circuit. Now referring to FIG. 6, a two stage Class A operation amplifier 6, incorporating the biasing, current mirroring, and common mode reconstruction circuitry techniques of the present invention, is illustrated. This amplifier 6 consists of a single stage amplifier section (like that of the device in FIG. 3), a network of DC biasing circuitry (like that of the device in FIGS. 3 and 4), and improved common mode reconstruction circuitry (like that of the device in FIG. 3). The second stages of the amplifier 6 are Class A output stages. The incorporation of the Class A output stages with the single stage amplifier section thus forms a generic two stage Class A fully differential operational amplifier. In this embodiment, transistors 413,414,415 and 433,434,435 and capacitors 425 and 426 form the output stages. Transistors 414,415 and 434,435 create constant current sources, while transistors 413 and 433 derive the output levels based on their respective gate potentials supplied from the first stage outputs 314,315. The circuitry sets up current sources which are biased from the same biasing sources 302,304 as in the single stage amplifier 4 design (FIG. 3). Capacitors 425,426 are standard Miller compensation capacitors. Transistors 413,433, in effect, act as "throttles" to modulate the outputs 416,418. As the gates of the respective transistors 413,433 go up, they put more drive on the respective transistors to pull the output down, and as the gates of the respective transistors 413,433 go down, they turn the device off and allow the output to pull up. Now referring to FIG. 7, those skilled in the art will recognize that numerous variations in the configuration and elements may be possible without departing from the functions of the device and the manner in which it accomplishes those functions. As an example of one alternative design, notwithstanding there are many others and so this example is not intended to be limiting, a pole-zero compensation, rather than the pole-splitting compensation described above, may be employed. This can be accomplished in the two stage amplifier 6 by replacing the capacitors 425,426 with series resistors 500,502 and capacitors 425,426. In other examples, further multiple stage, complementary arrangements, and other modifications, additions, and deletions may be made in the designs without departing from the intended scope of the description of the invention herein. In these and other embodiments based on the same principles described herein, the single stage amplifier is thought to be particularly applicable in higher frequency switched capacitor circuits (e.g., sigma-delta modulators, filters, etc. ) and the two stage amplifier is believed to have many diverse applications and to be suitable as an all-purpose operational amplifier. As is clearly seen, the present invention overcomes the problems presented by the prior art devices. The present invention is believed to be especially effective when configured and employed as described herein, however, those skilled in the art will readily recognize that numerous variations and substitutions may be made in the device and its use and configuration to achieve substantially the same results as achieved by the embodiments and, in particular, the preferred embodiments expressly described herein. Each of those variations is intended to be included in the description herein and forms a part of the present invention. The foregoing detailed description is, thus, to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being limited solely by the appended claims.

Claims: A fully differential amplifier including a common mode reconstruction circuit and a DC biasing network providing at least a first and a second bias voltage as well as an output stage bias voltage, characterised in that, in the DC biasing network, a first series connected structure (156,154), through which a first reference current (Iref) is drawn, comprises a first upper diode connected transistor (156) connected in series with a first lower transistor (154) in which the gate of the first lower transistor (154) is biassed by the first upper diode connected transistor (156), and a second series connected structure (152,150), through which a second reference current (Iref) is drawn, comprises a second upper transistor (152) and a second lower transistor (150) connected in series and wherein the gate of the second upper transistor (152) is biased by the first upper diode connected transistor (156) of said first series connected structure (156, 154) and wherein the gate of the second lower transistor (150) is connected to the drain of the second upper transistor (152), an output stage series connected structure (160,158), through which an output current (Iout) is drawn, comprises a third upper (160) and a third lower (158) transistor connected in series, wherein a first bias voltage (196) is derived from the said first upper diode connected transistor and is further used to bias the third upper transistor (160) of the output stage series connected structure (160, 158), and wherein a second bias voltage (198) is derived from the gate of the second lower transistor (150) of said second series connected structure (150, 152) and is further used to bias the third lower transistor (158) of the said output stage series connected structure (160, 158). The amplifier of claim 1 wherein the diode connected transistor (156) and the upper transistors ofthe said second and output stage series connected structures (152, 160) are sized the same or as unit multiples of one another. The amplifier of claim 2 wherein the lower transistors of the second and output stage series connected structures (150, 158) are sized the same or as unit multiples of one another. An amplifier according to any one of the claims 1-3 wherein the common mode reconstruction circuit is arranged to receive the first bias voltage (196), a reference voltage (242) and a mid point voltage (244) equal to the mid point of the voltages at first and second outputs of an amplifier section having an input stage and an output stage, the common mode circuit including means (218, 222, 224) for generating a first current in response to the reference voltage, means (220, 222, 224) for generating a second current in response to the mid point voltage, first current sink means (210, 214) for receiving the first current, and second current sink means (212, 216) for receiving the second current, the first current sink means providing a bias voltage to current sinking transistors (132, 136) in the output stage of the amplifier section in response to a difference between the reference voltage and the mid point voltage. The amplifier of claim 4, wherein the first bias voltage (196) is applied to the gates of cascode transistors (134, 138) connected to the respective sinking transistors (132, 136) in the output stage of the amplifier section, and to the gates of respective cascode transistors (214, 216) connected to respective current sink transistors (210, 212) forming the said first and second current sinking means of the common mode circuit. The amplifier of claim 4 or claim 5, wherein a first output stage bias voltage (202) obtained from a first (172) of a pair of series-connected transistors (172, 170) connected to the third upper transistor (160) of the output stage structure of the biasing network is applied to the gates of respective cascode transistors (122, 126, 130) connected to current source transistors (120, 124, 128) in the input and output stages of the amplifier section, and a second output stage bias voltage (204) obtained from the second (170) of the said pair of series-connected transistors is applied to the gates of the said current source transistors. The amplifier of claim 6, wherein the first and second output stage bias voltages (202, 204) are further applied to the gates of respective current source transistors (222, 224) in the common mode circuit, the current source transistors being common to the first and second current generating means of the common mode circuit. The amplifier of claim 5 wherein the gates of the respective sinking transistors (210, 212) in the common mode circuit are connected to the drains of the cascode transistors (214, 216) connected thereto. The amplifier of claim 7, wherein the current source transistors in the amplifier section and in the common mode circuit are p-channel transistors, and the current sink transistors in the amplifier section and in the common mode circuit are n-channel transistors.

Inventors: BREHMER GEOFFREY E, BREHMER, GEOFFREY E.
Applicants: ADVANCED MICRO DEVICES INC, ADVANCED MICRO DEVICES INC.
Application year: 1994
Main classification: H03F3
IPC codes: H03F3